Hard Shell jacket vs Rain Jacket

Wondering what the major differences are between the uber expensive hard shell ski-specific jackets, for example the Arcteryx Theta jacket and something that is more of a breathable rain jacket like for example the Mountain Hardware Quasar jacket?

Durability. Hard shell jackets have an extra layer of nylon, typically, and it's often thicker. If you fall on firm, or get a ski edge slash, makes a difference. Also, unless you're hiking or skinning, breathability at 15-20 degrees is not a biggie. Get something with pit zips.